edfas.org ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 24 NO . 4 24 layers is parallel to each other, the current passes through the tunnel, owing to its magnetic tunneling effect. The equivalent resistance state of the cell is considered a low state. On the other hand, the cell resistance will be high if the magnetic orientations of the layers are anti-parallel. Spin transfer torque RAM (STT-RAM) is considered one of the most successful emerging NVM candidates, owing to its unique characteristics like excellent scalability, high speed, lowpower consumption, andhighendurance capabilities. STT-RAM is a type of MRAM where spin-polarized currents are used. The STT-RAMmemory cell consists of an MTJ, a transistor, word line, bit line, and source line.[5] By applying spin polarized current, themagnetic orientation of the active information storage layer is changed, allowing it to write data. To read data, the difference in the MTJ element’s resulting resistance is utilized. Another prototype NVM, ferroelectric RAM (FeRAM), starts with a 1T-1C structure as DRAM. With an external electric field, atoms follow the applied electric field direction in the ferroelectric material, causing charge breakdown. The breakdown changes the polarization characteristics of the ferroelectricmaterials.[6] After removing the applied electric field, atoms retain a polarization state as the ferroelectric materials exhibit hysteresis. Though FeRAMhas faster write performance, lower power consumption, and excellent data reliability, it has less storage capacity in comparison to flash devices and is relatively expensive as well. More specifically, FeRAM can be divided into two subcategories: ferroelectric field effect transistor (FeFET) and ferroelectric tunnel junction (FTJ). The aforementioned 1T-1C structure memory issues can be overcome by integrating ferroelectric materials into the gate oxide, making 1T structure FeFET.[7] 1T FeFET provides low power consumption with as high performance as DRAM, and good CMOS compatibility. Meanwhile, FTJ works slightly differently as a two-terminal device. FTJ is composed of a ferroelectric layer between two metal electrodes, and an external electric field polarizes the ferroelectric materials. Phase change memories (PCM) have attracted large attention, owing to their good CMOS technology compatibility, long endurance, fast speed, and high scalability. In a PCM cell, a reversible chalcogenide phase changematerial is formed between two electrodes, and its resistance transition occurs between amorphous and crystalline phases, which determines the PCM performance. The reversible switching between the two resistance states can be achieved by applying a current pulse. When a short (tens of nanoseconds) and a high (hundreds ofmicroamps to milliamps) current pulse is applied, materials phase transform, or reset, from crystalline (low resistance) to amorphous state (high resistance). On the other hand, a relatively long (hundreds of nanoseconds) and low (100 to 200microamps) current pulse is applied to switchmaterials fromthe amorphous to the crystalline state, or set. The amorphous state can be considered as a high resistance state “1,” whereas the crystalline state is considered as a low resistance state “0.” Resistive random-access memory (ReRAM or RRAM) has also attracted large attention due to its compatibility with conventional semiconductor technology.[5] ReRAM is a simple, two-terminal, metal-insulator-metal device in which bistable resistance state oxide materials like NiO, ZrO2, HfO2 are used. Oxide materials switch between two or more distinct resistance states. It may form conducting filaments in the insulating oxide, creating oxygen vacancy when applying voltage to the electrodes. The ON-OFF switching between the low resistance state (LRS) and the high resistance state (HRS) is controlled by the filament formation and rupture process. Oxide-based resistive memory (OxRAM) is based on a change in resistance, caused by oxygen ion vacancymigration. Themost commonoxide-basedReRAM, filamentaryOxRAM, isbased on the valence change of the dielectrics between the electrodes. Conduction filament formation occurs across the dielectric by an electroforming process. Non-filamentary OxRAM is a resistive switching device comprised of one or more oxide layers. Though conductive filament is not needed, change in resistance is uniform for non-filamentaryOxRAM.[8] Field-driven redistribution of oxygen vacancies offers nonvolatilememory functionality by changing the electronic properties of the tunnel barrier. Finally, a conductive-bridge RAM (CBRAM) cell is a metal–ion conductor–metal (MIM) device with an electrochemically active electrode, comprised of Ag, Cu, or Ni ions. The ions migrate across insulating dielectrics, formingmetallic filaments. Applying bias voltages can change the resistance caused by the oxidation and reduction of the metal ions. In racetrack memory, hundreds of millions of nanowires areused to store vast amounts of data. TheU-shaped magnetic nanowire has a pattern ofmagnetic regionswith different polarities arranged vertically, offering the highest storage density.[4] The magnetization states of the magnetic domains are used as “0” and “1” bits. The magnetic information moves along the wire by applying voltage pulses to the wire ends. When applying a spin-polarized current, datamoves in either direction, depending on the direction of the current. Polymer memory is composed of a layer of organic polymer molecules, or nanoparticles, placed between
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